10.1 Introduction

In addition to pure fluctuations, waves can induce currents which do not change their directions during a time long in comparison with a wave period. Although the current velocity is often weak, its persistence can result in the transport of bottom sediments. In order to understand the changes of the sea bottom, it is necessary to acquire a good understanding of currents generated by waves.

Two different types of currents that owe their existence to wave fluctuations are of interest to us. In this chapter we examine the mass transport within the boundary layer near the sea bottom when the wave field above is essentially inviscid and irrotational. In the next chapter we shall examine the longshore current and its variations in and near the surf (breaking) zone of a gently sloping shore. In both cases the driving mechanism is the steady momentum flux due to the convective inertia of the waves, and dissipation plays a central role in maintaining the steady drift.

It was first discovered in acoustics that steady currents could be induced near a solid wall adjacent to an oscillating fluid. This phenomenon of acoustic streaming makes standing waves in tubes visible by the accumulation of dust particles at the nodes. Rayleigh (1883) first analyzed the phenomenon theoretically and determined Eulerian streaming in the boundary layer. Extension to water waves propagating in one direction was made by Longuet-Higgins (1953) who pointed out that Stokes drift had to be added to Eulerian streaming velocity...